1. Field of the Invention
This invention relates to a computerized tomography apparatus and more particularly to a computerized tomography apparatus having a relative large number of radiation detectors to produce output signals for developing an image representing a two dimensional slice through a patient's body.
2. Description of the Prior Art
In such an apparatus as above noted, a radiation source rotates around the patient within a selected plane and a fan-shaped beam is irradiated through the patient at many different views so that the beam passes through the slice specified by the plane. The x-rays of different paths through that slice are absorbed to a greater or lesser degree depending on the absorbitities of the patient's tissue through which the x-rays pass. The intensities of the x-rays emerging from the body represent the absorption coefficiencies integrated along the paths followed by the x-rays. An array of detectors positioned opposite the fan beam source beyond the patient's body measures the intensity of the emerging radiation and produces output signals corresponding to the measured radiation intensities. These signals are processed by a reconstruction unit such as a device described, for example, in U.S. Pat. Nos. 4,149,249 and 4,351,247 to produce rapidly a two dimensional image of the body slice.
Normally each detector includes a scintillation crystal upon which radiation impinges. A radiation photon causes the scintillation crystal to luminescence with an intensity related to the intensity of the radiation. The light resulting from the luminescence diverges in all directions. A portion of the light experiences a reflection against a reflection layer surrounding the side wall of the crystal and descends toward the bottom of the crystal. Each detector also has a photoelectric transducer disposed at the bottom of crystal and optically connected with each crystal. The light which reaches the transducer is converted into a electrical signal indicative of the intensity of luminescence, i.e., the intensity of radiation impinging upon the scintillation crystal. However, the above described reflection and long path travelled by the reflected light causes a certain loss of optical energy. Also each detector includes a partition opaque to radiation juxtaposed between adjacent crystals to prevent incident radiation and scattered radiation from producing cross talk between the adjacent crystals.
To obtain a high resolution image of the body slice, it is essential that each detector is arranged with as small pitch as possible relative to the fan-beam emerging from the body in order to increase the sampling pitch of the detector array. Further, it is necessary to minimize the dimension of crystal in the fan-beam direction. Otherwise, there occurs a problem that the output signal of detector is reduced because the light receiving area of the photoelectric transducer is diminished. In that event, it is necessary to expose the patient to a larger radiation dosage in order to collect enough information for image reconstruction.